The use of the word cúige, earlier cóiced, literally "fifth", to denote a province indicates the existence of a pentarchy in prehistory, whose members are believed to have been population groups the Connachta, the Ulaid (Ulster) and the Laigin (Leinster), the region of Mumu (Munster), and the central kingdom of Mide. This pentarchy appears to have been broken up by the dawn of history in the early 5th century with the reduction of the Ulaid and the founding of new Connachta dynasties which expanded north and east.
Medieval Irish historical tradition traces these dynasties to the four or five sons of Eochaid Mugmedon: Brion, Ailill, Fiachrae, Fergus Caech (perhaps a literary addition), and Niall of the Nine Hostages. Four were ancestors of new Irish dynasties; those of Brión (the Uí Briúin), Fiachrae (the Uí Fiachrach) and Ailill (the Uí Ailello, later replaced by Uí Maine) were known as teóra Connachta, or the historical Three Connachta of the province itself; that of Niall, the Uí Néill, at first surpassed its parent dynasty, establishing or continuing the so-called High Kingship of Ireland at Tara, and became the most powerful dynasty in Ireland down to early modern times.
However David Sproule points out that:
It does not seem that the word "Connacht" can originally have meant 'the descendants of Conn'; it may have meant 'headship' or 'supremacy' from "cond" or "conn", head, and later have been interpreted as meaning "the descendants of Conn", Conn Cetchathach being derived from the word "Connacht" rather than vice versa. ... the name "Eoganacht" and "Ciannacht" were formed in imitation ...
Sproule's hypothesis has been accepted by historians such as Paul Byrne.